3-phase AC mains supply is typically used for distributing generated power to end consumers. In a 3-phase system each AC supply is shifted by 120° (2π/3 radians) with respect to the other phases. One advantage of 3-phase power systems is the ease with which rotating machinery can generate the necessary phase shifted AC currents.
An important requirement when loading a 3-phase circuit is that each phase is loaded equally; in other words the loads on each of the 3 phases are roughly equivalent. When the loading on the phases are roughly equivalent the loading is said to be balanced. Because the phase currents are 120° out of phase the instantaneous sum of the currents for a balanced load is zero.
When the phase loading is balanced the system losses are reduced and lower rated conductors can be utilised. Unbalanced loads are also undesirable for generation. In a 3-phase generator the phase windings are typically distributed around the stator, spaced apart by 120° (2π/3 radians). When phase loads are unbalanced (phase currents varying more than 10%), rotor overheating may result in the generator. The rotor heating is caused by negative sequence currents in the field pole face.
Furthermore, when one of the phase windings is more heavily loaded than the others the stator experiences a greater resistance to its rotation when inducing current into the heavily loaded winding versus a less heavily loaded winding. The differing loading on the stator results in mechanical vibration, stress and accelerated wear and tear on the generator.
In addition to undesirable mechanical stresses an unbalanced load can cause problems with automatic voltage regulation (AVR) circuitry. An AVR circuit regulates the output voltage of the phases at a desired level, for example 110 VAC or 230 VAC. When an unbalanced load is applied to a generator the AVR circuitry attempts to maintain the voltage on each phase at the desired level, however due to one phase being heavily loaded one phase may be disproportionally lower than the voltage on the other phases, this can result in instability of the AVR circuitry.
Unbalanced loads on a generator can cause load imbalance alarms and generator shut-down. Due to the issues caused by unbalanced loads utility power providers can charge penalties to consumers of 3-phase power who do not balance their demand across all phases.
Typically issues of unbalanced loads are avoided by consumers by ensuring that the loading on each phase is equal, usually by installing phase balancing circuitry. However typical phase balancing circuitry is extremely expensive. Traditional phase balancing circuitry is not well suited to rapidly changing loads, such as when loads are spasmodic, or unpredictable. Examples of such loads include automated compressors or pumps such as would be found in air-conditioning and refrigeration equipment.
Traditional phase load balancing systems utilise equipment that automatically switches AC loads from phase to phase to try to attain a balanced load. This generally involves switching equipment off momentarily as it transfers to a different phase, and the load steps are limited by the actual loads attached, and as such the balancing may be relatively coarse.
The most common technique for balancing loads is to simply distribute single-phase loads evenly across phases at the time of design. This does not however address the problem of single phase loads turning on and off.
It would be useful for a system to exist in which the individual loading can be varied dynamically to adjust for spasmodic or unpredictable loading.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
Throughout this specification, the word “comprise”, or variations thereof such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.